The invention relates generally to liquid crystal displays, and, more particularly, to a liquid crystal display having improved isocontrast performance and a method for producing same.
Liquid crystal displays (LCDs) have been in use for quite some time and are useful for displaying information. Although LCDs are attractive for use in displays for portable devices in which power consumption is a concern, LCDs are also commonly used in other display applications, such as test and monitoring equipment. Advantageously, LCDs typically consume a relatively small amount of power.
An LCD can be fabricated either as a reflective display, in which light passes through the display and is reflected back through the display for viewing, or as a transmissive display, in which a light source is located behind the display and a viewer directly views the display.
FIG. 1A is a cross-sectional view illustrating an example of a conventional reflective LCD 10. Briefly described, an LCD is fabricated by locating a liquid crystal (LC) material 11 between glass substrate elements 16 and 17, to which transparent indium tin oxide (ITO) electrode lines 12 and 14 have been applied. The ITO lines 12 and 14 are applied to the glass substrate elements 16 and 17 in a manner such that the ITO lines form a matrix of intersecting lines. The ITO lines 12 and 14 are typically oriented orthogonal to each other such that a picture element (pixel) is formed in the liquid crystal material at the intersection of each ITO line 12 and 14.
A polarizer 18 is applied to a surface of glass substrate element 16 opposite that to which the ITO line 12 is applied. Similarly, analyzer 19 is applied to a surface of glass substrate element 17 opposite to which the ITO line 14 is applied. A diffuser 21 is applied over the analyzer and a reflector 22 is applied over the diffuser. In the case of a transmissive display, a light source is located in place of the reflector 22. The polarizer 18 is a thin film applied to the glass to serve as a filter that polarizes the impinging light so that the entering light beam is polarized in one direction. The analyzer is a thin film that aids in the polarization process. The diffuser 21 is also a thin film that diffuses, or smears, the light beam so that annoying birefringence does not occur when the display is viewed. Birefringence can be explained by understanding the refraction of a plane wave of light at the boundary between an isotropic medium (such as air) and an anisciropic medium (such as a crystal). The wavefronts of the incident wave and the refracted wave should be matched at the boundary. Because the anisotropic medium supports two modes of distinctly different phase velocities, for each incident wave there are two refracted waves with two different directions and different polarizations. This effect is known as birefringence.
Upon the application of an electrical potential between the ITO line 12 and the ITO line 14, an electric field is established between the ITO lines 12 and 14 and passes through the LC material. In accordance with known principles, the molecules in the LC material, in response to the electric field, become mobile and. depending upon the type of LC material, will rotate, twist, or otherwise change state, thereby preventing light, by the cross polarizing of the traveling light wave. from passing through the display and appearing dark to a viewer. The display may be normally white or black. Upon the application of the electric field, the LC material will change state. In other words, if the material is xe2x80x9cblackxe2x80x9d it will become xe2x80x9cwhitexe2x80x9d and if the material is xe2x80x9cwhitexe2x80x9d it will become xe2x80x9cblack.xe2x80x9d Importantly, the LC material changes state in response to the electric field applied by the ITO lines 12 and 14.
FIG. 1B is a plan view schematically illustrating a conventional pixel 15 formed at the intersection 23 (referred to as a xe2x80x9cpixel junctionxe2x80x9d) of ITO lines 12 and 14 of the LCD 10 of FIG. 1A. Some of the elements have been omitted for clarity. Pixel 15 includes the LC material 11 located at the intersection of, and disposed between ITO lines 12 and 14.
FIG. 1C is a cross-sectional view of the conventional pixel 15 of FIG. 1B. In response to the electric, or e field 25, created in the region of the pixel junction 23 between ITO line 12 and ITO line 14, the molecules that make up LC material 11 will change state, or rotate, thereby becoming visible to a viewer (24 of FIG. 1A).
FIG. 1D is a cross-sectional view of the conventional reflective LCD 10 of FIG. 1A illustrating the difference between an xe2x80x9caddressedxe2x80x9d pixel and a xe2x80x9cnon-addressedxe2x80x9d pixel. In FIG. 1D, the LC material 11 is illustrated as comprising individual molecules, an example of which is indicated by reference numeral 13. The voltage source xe2x80x9cVsxe2x80x9d 27 corresponding to pixel 33 indicates that the LC material 11 within pixel 33 is selected or addressed. When addressed, the orientation of the individual molecules 13 within the LC material 11 sandwiched between alignment layer 28 and alignment layer 29 change state, or twist, and appear to xe2x80x9cstraighten outxe2x80x9d. Alignment layers 28 and 29 are each thin films which have been physically rubbed in specific directions so as to assist the LC molecules 13 adjacent to these layers to pre-rotate in favorable directions. For example, if it is desirable for an LCD to have a preferred viewing angle, these rubbed layers enhance that angular view. The aligned molecules 13 (associated with pixel 33) allow the light from light source 24 to pass through the LC material 11 with a specific polarization. The light from light source 24 can be reflected back to the viewer 26 through glass substrate element 16 and polarizer 18.
The molecules 13 within pixel 35. associated with voltage source xe2x80x9cVnaxe2x80x9d 26, have not been addressed. The random molecular orientation of these molecules 13 suppresses the light from light source 24 and prevents the light from passing through the LC material 11 associated with pixel 35. Hence, pixel 35 is non-addressed and would appear dark to a viewer 26.
FIG. 1E is a graphical representation 31 of the isocontrast curves of pixel 15 of FIGS. 1B and 1C. When LCDs are viewed at angles normal to, or nearly normal to, the surface of the LCD display, the rotated liquid crystal material is easy to discern. However, when viewed at off angles, the polarizing effect of the twisted liquid crystal material on the traveling light wave quickly becomes indiscernible. This is caused by the crystalline nature of the liquid crystal material. This condition is illustrated in FIG. 1E, which is a graphical representation of a contrast curve (referred to as an isocontrast curve) for a conventional pixel 15. A contrast curve which has the same contrast ratio (light returning from the addressed pixel/light returning from a non-addressed pixel) at every point on its curve is called an isocontrast curve. As shown in FIG. 1E, the liquid crystal material in the region of pixel 15 clearly has better contrast at some angles that at other angles. For example, isocontrast line 34 shows that the pixel has a higher contrast when viewed at approximately 180 or 360 degrees, than it does when viewed at 90 or 270 degrees. For example, arrow 37 indicates a viewing angle in which a viewer would see limited contrast.
Therefore there is a need in the industry for a liquid crystal display in which the contrast of the liquid material may be controlled and maximized depending on the viewing angle desired.
The invention is a liquid crystal display having improved and controllable isocontrast and a method for producing same.
In architecture, the invention can be conceptualized as a liquid crystal display, comprising a liquid crystal material disposed between a pair of transparent plates. The display includes a first electrical conductor and a second electrical conductor associated with the liquid crystal material and configured to form a picture element in an overlap region in which the first electrical conductor and the second electrical conductor overlap. The first electrical conductor and the second electrical conductor are configured to apply an electric field to the liquid crystal material. The electric field causes molecules in the liquid crystal material to change state in response to the electric field in the overlap region associated with the picture element. The overlap region is selectively defined to alter the electric field so that a degree to which the liquid crystal molecules change state in response to the electric field is controlled by the selectively defined overlap region.
The invention can also be conceptualized as a method for controlling contrast in a liquid crystal display, the method comprising the steps of: forming a liquid crystal material between a pair of transparent plates and associating a first electrical conductor and a second electrical conductor with the liquid crystal material. The method also includes the step of forming a picture element in a region in which the first electrical conductor and the second electrical conductor overlap. The first electrical conductor and the second electrical conductor apply an electric field to the liquid crystal material. The electric field causes molecules in the liquid crystal material to change state in response to the electric field in the region associated with the picture element. The method also includes the step of selectively defining the overlap region to alter the electric field so that a degree to which the liquid crystal molecules change state in response to the electric field is controlled by the selectively defined overlap region. The invention allows control over the shape of the electric field so that the change in state, or twist, of the liquid crystal material is controllable so as to allow favorable viewing of the display from any angle, thereby reducing, and possibly eliminating, blind spots.
An advantage of the invention is that it allows control over the contrast of a liquid crystal display.
Another advantage of the invention is that it is simple in design and easily implemented on a mass scale for commercial production.